Page 46 - Cardiac Nursing

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22 PA R T I / Anatomy and Physiology

■ Figure 1-20 Characteristic action potentials in dif-
ferent regions of the heart. See text for description.
(From Katz, A. [2006]. Physiology of the heart [4th ed.,
p. 414]. Philadelphia: Lippincott Williams & Wilkins.)

period depends on the time it takes to remove inactivation from lowed for ventricular filling depend on normal cardiac electrical
the sodium and calcium channels. The effective refractory period refractoriness.
extends from phase 0 through the middle of phase 3.
During the relative refractory period, only a stimulus greater
than normal can initiate an action potential. The relative refrac-
tory period occurs during the latter part of repolarization (late SARCOLEMMAL IONIC
phase 3). Under certain conditions, a stimulus can initiate an ac- CURRENTS
tion potential during the last part of phase 3 and the beginning of
phase 4. Cardiac arrhythmias may occur during when this hap- The currents that combine to orchestrate the action potential can
pens, especially when pathophysiological situations, such as is- be studied independently. Neurohormonal and ionic milieu,
chemia, promote abnormal refractory periods. pharmacologic agents, and pathologic processes variably influence
The entire period between depolarization and complete repo- each current. Techniques such as the patch clamp and molecular
larization is termed the full recovery time. Under normal condi- biology have extended our understanding of channels types and
tions, cardiac cells are not depolarized until they have had time hold the promise of increasing our understanding of important is-
to recover fully from the previous depolarization. Usually, cells sues such as the generation of arrhythmias in disease states. New
with long refractory periods have long action potential durations. treatments are being developed. Some of the major channels are
The upper limits of normal heart rate responses and the time al- discussed individually (Table 1-4).

■ Figure 1-21 Refractory periods. Closing of the h gates immediately after membrane depolarization causes
the absolute refractory period, during which no stimulus regardless of its strength is able to initiate a propa-
gated action potential. This is followed by a relative refractory period (RRP) during which only stimuli that ex-
ceed the normal threshold can cause a propagated action potential. The functional refractory period, which in-
cludes the absolute and relative refractory periods, is followed by a supernormal (SN) period, during which
subthreshold stimuli slightly less than those that reach the normal threshold can generate a propagated action
potential. Action potentials generated during the relative refractory and supernormal periods are small and
slowly rising because of incomplete recovery of the sodium channels. The full recovery time begins with depo-
larization and ends after the supernormal period, when normal stimuli produce normally propagated action po-
tentials. (From Katz, A. [2006]. Physiology of the heart [4th ed., p. 397]. Philadelphia: Lippincott Williams &
Wilkins.)